The invention relates to a telecommunications network organized in wavelength-division-multiplexed optical loops. It can be applied to any type of telecommunications network, namely to public or private transmission networks. Thus, it can be applied to the national and regional telecommunications networks, local networks, and data communication networks.
In the field of telecommunications, the term "optical loop" refers to a set of stations connected to one another by one or more optical fibers constituting the means for conveying information from one station to another. These stations form variable traffic nodes. The term "wavelength-division-multiplexed optical loop" refers to an information transfer loop wherein a station transmits information elements to another station of the loop on a given wavelength assigned to this station, each station having a predetermined reception wavelength.
SDH (Synchronous Digital Hierarchy) networks using transmission on optical fibers are organized in loops which may have two types of architecture, a one-directional architecture as shown schematically in FIG. 1a or a bi-directional architecture as shown schematically in FIG. 1b.
The one-directional loop (FIG. 1a) has a station at the "head" of the network connected to several stations in a clearly defined direction. Each station can extract the information elements intended for it and insert other information elements which will be transmitted to the head of the network. The insertion/extraction or add/drop function is achieved by an add/drop multiplexer (ADM).
The bi-directional loop (FIG. 1b) is distinguished from the one-directional loop by the absence of a head station or central station for each station is connected to all the other stations of the loop (by a multiconnection or meshed network). This is provided by two distinct fibers where the information elements flow in opposite directions. Furthermore, this type of loop provides for greater security because two backup fibers are used in addition to the two operational fibers.
The transfer of information elements in the loop is done according to the hierarchy of the SDH or SONET standard by means of a frame which is the STM4 or STM16 frame depending on the number of stations of the loop. Thus, as soon as the number of stations is greater than 4, the frame used is an STM16 frame working at 2.5 Gbit/s obtained by an electrical time-division multiplexing of 16 STM1 frames at 155 Mbit/s.
It is observed that when the number of stations is between 4 and 16, for example 7, the STM16 frame has 9 empty frames. It is therefore observed that the use of a frame at 2.5 Gbit/s is not optimized in every case. Furthermore, the instruments, especially the transceivers for a 2.5 Gbit/s transmission, are very costly.
With the recent introduction of parallel wavelength-division-multiplexed networks, namely wavelength-division-multiplexing, in these looped network architectures, an additional degree of flexibility has been obtained in enabling optical continuity to be maintained through the loop. This will be understood more clearly from the article by A. F. Elrefaie, Multiwavelength Survivable Ring Network Architectures, ICC 93, Geneva.
For wavelength-division-multiplexing (WDM), optical frequency add-and-drop multiplexers based on acousto-optical or Fabry-Perot filters as well as integrated optics waveguides have been used.
Reference could be made to the following articles in this field:
M. Fukutoko et al., Wavelength-division-multiplexing Add/Drop Multiplexer Employing A Novel Polarization Independent Acousto-Optical Tunable filter, in ELECTRONICS LETTERS, Vol. 29, pp. 905-907, 1993, PA1 K. Oda et al., An Optical EDM-Add/Drop Multiplexing Ring Network Utilizing Fiber Fabry-Perot Filters And Optical Circulators, in IEEE Photonics Technology Letters, Vol. 5, No. 7, 1993, PA1 Y. Tachikawa et al., Arrayed-Waveguide Grating Add/Drop Multiplexer With Loop-Back Optical Paths, in Electronics Letters, Vol. 29, pp. 2133-2134, 1993.
Only one wavelength is used per station for transmission and reception and this wavelength is fixed once and for all. The principle of these different architectures relies on the elimination (or rejection) of one wavelength from each station during the information dropping or extraction operation in order to replace it during the information adding or insertion operation by another wavelength having the same value.
The other drawback related to the fact that one wavelength (this wavelength being fixed beforehand) is used per station is that it is not possible, with this technique, to share the bit rate among several stations.
Now, the Applicant has noted that if the traffic between two or more stations is low, then it would be worthwhile to share the data elements among these stations. More generally, it would be preferable to provide the transmission on such networks with greater flexibility. This flexibility may be obtained according to the invention by enabling a reconfiguration of loops according to traffic or according to the malfunctions that occur in the loops of the network.